Mio Aoyama is a supporting character in The World God Only Knows and the second capture target in the series. She is a sixteen-year-old student at Maishima Academy, where she is initially perceived as a wealthy heiress. In truth, her father, the president of the Aoyama Corporation, died over a year before the events of the story, and her family has lost its fortune. She now lives in a run-down apartment, struggling to maintain the appearance of affluence she once had. She has blond hair styled into twin-tails, with red hairpins to keep her hair off her face, and she wears high-heeled shoes despite being short in stature, likely as a way to compensate for her complex about her height.

Her personality outwardly matches many traits of a classic tsundere archetype, as Keima Katsuragi notes based on her cat-like eyes, light hair color, visible forehead, and pigtails. At first, she is proud, rigid, and somewhat haughty, but this masks deep-seated guilt and shame stemming from her father's death and her family's fall from grace. She was raised by her father to always live with pride, and she holds herself to those high standards even after losing everything, believing that deviating from that role would dishonor his memory. Consequently, she punishes herself with immense internal pressure and judges others harshly when they do not meet similar expectations. There is also a playful, sadistic edge to her interactions with Keima, teasing him in a way that reflects her tsundere nature.

Her primary motivation is to preserve the dignity and memory of her father, even as she struggles to maintain the illusion that her family is still wealthy. This drive makes her fiercely independent, though it also isolates her. Her role in the story begins as a target for Keima to capture in order to expel a loose soul. During this process, Keima must break through her pride and help her confront the truth of her situation, ultimately allowing her to accept her new reality and let go of the pretense. She later appears as a supporting character in the goddess arc, where her heart becomes one of the vessels for a goddess, though she is not one of the primary hosts discovered early on.

Her key relationships include her deceased father, whose teachings and example define her entire worldview, and Keima Katsuragi, who becomes the catalyst for her emotional breakthrough. Through Keima's strategic efforts, she is able to reconcile her pride with her genuine feelings, and she develops a grudging respect and affection for him, although the memories of that conquest are removed afterward. She also interacts with other capture targets and classmates, but her primary connection is to Keima and the lingering influence of her father.

Over the course of her arc, Mio experiences noticeable growth. She begins trapped in a cycle of denial and self-imposed pressure, but after being confronted with her inability to maintain the lie, she learns to accept her reduced circumstances without losing her sense of self-worth. She becomes more honest with herself and others, though she retains her characteristic pride and strong-willed nature.

In terms of notable abilities, Mio does not possess any supernatural skills or special combat prowess. Her strengths lie in her resolute willpower and her capacity for self-discipline, both of which are products of her upbringing. She is also physically agile, a trait that becomes briefly relevant during the chase sequences of her capture arc. Her most significant ability, if it can be called that, is her ability to project an image of unshakable confidence even when her world has crumbled, which Keima identifies and exploits in order to reach her true feelings.